Slaven Bilic has been sacked as West Ham manager with the team in the Premier League relegation zone.
Bilic's last game in charge was Saturday's 4-1 defeat by Liverpool at London Stadium.
A statement from West Ham said the club believed a change was necessary "to move forward positively and in line with their ambition."
Former Everton and Manchester United boss David Moyes has been strongly linked with succeeding Bilic.
West Ham say Bilic's coaching assistants Nikola Jurcevic, Edin Terzic, Julian Dicks and Miljenko Rak have also left the club with immediate effect.
"The club's search for a new manager to take West Ham United forward is under way and an announcement regarding Bilic's successor is expected to be made over the coming days," added the statement.
Former Croatia defender Bilic, 49, was appointed in the summer of 2015 and guided West Ham to seventh place in his first season in charge.
But an 11th-placed finish last term, their first at London Stadium, has been followed by a poor start this season.
The Hammers have won just two Premier League matches in 2017-18 - and lost their first three league matches of the campaign.
Speaking after the defeat by Liverpool, Bilic told BBC Sport: "The pressure mounts game by game and at this moment it is big.
"I don't feel a broken man. On the other hand, the situation for West Ham is not good and the club is above every individual.
"I cannot say anything negative [about the owners]. They have been supportive, very good to me in my two and a half seasons, but the situation is very difficult."
West Ham's next match is at Watford on Sunday, 19 November.